The present invention relates to a steering control system for hydrofoil craft, and more particularly to a control system providing improved handling qualities and maneuverability during takeoff of such craft.
Hydrofoil craft have foils which are attached to the hull by struts and which move through the water below the surface when the craft is operated in the foil-borne mode. The foils develop lift in much the same manner as an aircraft wing and when a sufficiently high speed is attained they support the hull of the craft above the surface of the water. The craft is controlled by control surfaces or flaps pivotally mounted on the foils, or the foils themselves may be pivotally mounted on the struts to function as control surfaces, and a rudder is also provided for steering the craft. At low speeds, or when the struts are retracted to raise the foils from the water, the craft floats on the surface and operates in the hull-borne mode in the same manner as any other watercraft. When the struts are extended into the normal operating position and the craft is accelerated, lift is developed, as mentioned above, and when the craft has accelerated to a sufficient speed, typically between 30 and 40 knots, the hull is lifted above the surface and supported by the struts as long as the speed is maintained. During operation in this mode the control surfaces are automatically controlled in response to signals derived from suitable sensors and other control devices, and are positioned to maintain the desired attitude and direction of the craft and its height above the water to control and stabilize roll, pitch and yaw of the craft. A control system of this type is shown, for example, in Stark et al U.S. Pat. No. 3,886,884.
In the control system of the Stark et al patent, steering of the craft is controlled in response to command signals from a helm which may be either automatically controlled or controlled manually by the pilot. The signals generated by the helm when a turn is to be made actuate the aft control surfaces to rotate in opposite directions which causes the craft to bank about its roll axis in the direction of the desired turn. This rolling movement results in actuation of the rudder to make the turn and to achieve the desired turn coordination. The rudder could also, of course, be actuated directly in response to the helm command signals, if desired. It has been found in the use of this system that it is difficult to obtain good handling qualities and maneuverability during takeoff of the craft, that is, during the period of acceleration from the hull-borne mode to the foil-borne mode. Good maneuverability is particularly important during takeoff because it frequently occurs in locations such as harbor channels, where room to maneuver is limited and where traffic may be quite heavy so that rapid response to the helm and good maneuverability are important.
Both rate of turn and coordination of turns are important to good handling. It is desirable to have a relatively high rate of turn of the craft available when needed, and good coordination of turns is also highly desirable for good maneuverability and control. Coordination refers to the relation of the rotational, rolling, gravitational and other forces acting on the craft during a turn and can best be expressed in percent. Thus, 100% coordination refers to a condition in which the normally vertical resultant force acting on the craft maintains its perpendicular relation to the transverse axis of the craft during a turn even though the transverse axis itself departs substantially from the horizontal. Coordination is important not only to minimize passenger discomfort during a turn, but also because highly over- or under-coordinated turns increase the probability of strut ventilation. This is a condition in which the strut actually separates from the water, creating a vacuum or low-pressure space adjacent the strut which adversely affects the control and maneuverability of the craft.